1. Field of the Invention
The present invention relates to a method and an apparatus for measuring a depth of a hole in a composite-material workpiece machined by a rotating cutting tool orbiting about a principal axis. The present invention is, in particular, useful when making holes in multilayered workpieces, for example in various fiber-reinforced composite materials, laminates or material stacks including at least two layers material for use in the aircraft and space technologies.
2. Description of the Related Art
In aircraft multilayered components the thickness thereof may vary along the extent of the workpiece and thus the depth of the holes being formed therein may vary accordingly. It is important to accurately determine the individual depth of each hole being formed so as to be able to properly match the hole with an individual, accurately fitting fastener, such as a bolt or a blind fastener. Also, it is desirable to minimize the axial movement of the cutting tool in order to avoid unnecessary drilling in the air after having penetrated the workpiece.
A previous method of detecting the initial contact between a cutting tool and the surface of the workpiece and to determine when the cutting tool reaches various interfaces and depths of a multilayered workpiece is disclosed in the U.S. Pat. No. 4,644,335. This method involves the steps of detecting and analyzing output signals as to the acoustic signatures thereof, e.g. waveforms, amplitudes and frequencies of the output signals. However, such measuring methods are sensitive to disturbances and interferences caused by and transferred from mechanical working operations being performed simultaneously on other locations of the same workpiece. This may jeopardize the reliability of the measuring results of the depth in the respective hole.
What is needed in the art is a method and apparatus capable of determining the exact initial position of contact (a first zero reference position) between an axial cutting edge of a cutting head of the cutting tool and a first surface of the workpiece and to determine the exact position when radial cutting edges of the cutting head has fully penetrated the opposite, second surface of the workpiece.